1. Field of the Invention
The present invention relates to a method for manufacturing a plated steel sheet used for printed cans such as food cans and drink cans.
2. Description of the Related Art
Cans such as food and drink cans used for canned food or cans used for protecting sweet stuffs or dried goods against humidity are made of a material such as a tin-plated steel sheet, a chromium-plated steel sheet, or an aluminum sheet. Most such cans have printing on their outer surfaces to obtain a good outer appearance. In particular, a demand for cans used for gifts or souvenirs depends on designs, colors, and color tones of the cans. That is, a good outer appearance is important for these cans.
In order to obtain a good outer appearance, a white coating is formed on the surface of a steel sheet to cover a color inherent to a metal sheet so that colors can be painted thereon, or printing is performed on the surface of a steel sheet using a transparent ink to utilize the color or gloss of the metal. In either method, the higher the image clarity of the plated steel sheet surface is, the clearer and more beautiful the printed colors become. Since low image clarity of a steel sheet allows only dark colors, the image clarity on the surface of a steel sheet must be high with any method used.
A conventional plated steel sheet for cans is obtained by forming an electroplated layer on the surface of a steel sheet having a surface roughness Ra (average central roughness) of 0.12 to 3.0 .mu.m. The image clarity of this steel sheet is, however, unsatisfactory. No steel sheet capable of solving this problem has been developed.
As for steel sheets for applications other than cans, e.g., for decoration, bright chromium plating, bright nickel plating, and the like have improved the image clarity of a steel sheet. In these methods, however, a suitable plating condition range is narrow, and complicated treatment steps are required. Therefore, these methods cannot be used for can materials which must be mass-produced at low cost. Japanese Patent Disclosure (Kokai) No. 60-67653 discloses a method in which a paint is coated on a steel sheet to flatten its surface and then a metal is dry-plated thereon. This method is, however, not suited to can materials because electrical welding or mass production cannot be performed.